Seismic reflection method using an acceleration sensor is executed in the field of underground resource exploration. The seismic reflection method is a type of geophysical exploration, and is a method of artificially generating elastic waves, capturing reflected waves rebounded from underground by geophones placed on the surface of the earth, and clarifying an underground structure by analyzing the result.
In the seismic reflection method, elastic waves are excited to the ground from a vibration source placed on the surface of the earth, and the elastic waves reflected at the boundaries of strata are sensed by a geophone placed on the surface of the earth. Elastic waves excited in various directions propagate into the ground with large attenuation, are reflected at a plurality of strata, propagate again into the ground with large attenuation, diffuse to a wide region, and return to the surface of the earth. Therefore, the acceleration sensor used for the seismic reflection method needs to detect acceleration which is applied in the vertical direction, that is, in the same direction as of the gravitational acceleration, and is smaller than gravitational acceleration. That is, in the acceleration sensor used in the seismic reflection method, it is necessary to improve the sensitivity of acceleration in the vertical direction.
Such an acceleration sensor may include a substrate, a fixed portion fixed to the substrate, and a movable portion with one end portion connected to the fixed portion, and an electrode disposed to face the movable portion, and detecting acceleration, based on a capacitance between the movable portion and the electrodes.
Pamphlet of International Publication No. 2010/122953 (PTL 1) discloses a technique for an acceleration sensor which is a micro electro mechanical systems (MEMS) element, and the acceleration sensor includes a substrate, a fixed portion fixed to the substrate, a movable portion provided with a gap from the substrate, and a support beam connecting the movable portion and the fixed portion. JP-A-2001-272415 (PTL 2) describes a technique for a detection device for a differential type semiconductor acceleration sensor, which includes two sets each having a movable electrode and a fixed electrode, and detects acceleration, based on a difference in respective values of capacitance generated between both electrodes. U.S. Pat. No. 6,497,149 (PTL 3) describes an accelerometer including a pair of fixed electrodes, and a movable electrode, and uses a change in a capacitance in order to detect mass movement.